CVE-2026-64298
HighCVSS 7.1Exploitation Probability (EPSS)
Low risk6th percentile - higher than 6% of all known CVEs
Summary
In the Linux kernel's NFSv4 client, an access authorization flaw exists for O_TRUNC operations. The nfs_open_permission_mask() function does not include O_TRUNC in the permission mask, allowing local file open with read-only mode and simultaneous truncation, violating POSIX.
Risk Assessment
POSIX violation may allow a user to truncate a file they lack write permission for, potentially leading to data loss or privilege escalation.
Recommendation
Update the Linux kernel to a version that includes the fix adding MAY_WRITE to the permission mask when O_TRUNC is set.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: NFSv4: include MAY_WRITE in open permission mask for O_TRUNC POSIX requires write permission to truncate a file, so an open() that specifies O_TRUNC must be authorized for write access regardless of the O_ACCMODE access mode. nfs_open_permission_mask() builds the access mask passed to nfs_may_open(), which is the local authorization gate for OPENs the client serves itself from a cached write delegation via the can_open_delegated() path in nfs4_try_open_cached(). The mask is derived from O_ACCMODE alone, so an open(O_RDONLY | O_TRUNC) against a file the caller cannot write requests only MAY_READ and passes the local check. The OPEN is then satisfied locally and the truncation is issued to the server as a SETATTR(size=0) over the delegation stateid, which the server accepts under standard write-delegation semantics. POSIX requires that this open fail with EACCES. Include MAY_WRITE in the mask whenever O_TRUNC is set so the local check matches the access the server would have enforced.